The criterion according to which the best mechanism of a complex process and the associated kinetic parameters correspond to the best fitting of TG and DTG curves recorded at several heating rates is discussed. The main problem is if the application of such a criterion for a complex process will lead to the mechanism and corresponding kinetic parameters to be used for predictions. To this end, the thermal degradation of HDPE has been investigated by TG/DTG + DSC simultaneous analysis performed in Ar flow, at five heating rates as well as in four quasi-isothermal conditions. An algorithm for correct determination of the mechanism and the corresponding kinetic parameters of a complex process from non-isothermal and quasi-isothermal data was suggested. (C) 2009 Elsevier BM. All rights reserved.